This project will examine the development of physiological measures and sleep states in normal infants and in infants at risk for the sudden infant death syndrome (SIDS). The objective is to find physiological characteristics that differentiate infants who later succumbed from those who survived, and apply these characteristics to a yet-unexamined group at risk for SIDS. Infants who later succumbed to SIDS, and two groups of infants at increased risk of SIDS, siblings of SIDS victims and infants who experienced a life-threatening event, will be compared with age-matched controls using data already collected. The objectives are based on the assumptions 1) that SIDS victims succumb from a failure of mechanisms that normally allow a state transition when the infant is exposed to a life- threatening challenge, and 2) that physiological signs, measured during different states, will provide suggestions of the mechanisms that lead to a failure to respond adequately to challenges. Instantaneous and long term changes in cardiac and respiratory intervals, obtained during each sleep- waking state, will be assessed over the first 6 months of life in normal infants and AIDS-siblings, and in single recordings of infants who experienced a life threatening event or subsequently died of SIDS. The interaction between physiological variables that define sleep states will be assessed using linear time-domain filtering procedures and frequency- domain spectral techniques, together with nonlinear dynamic procedures including phase-plane plots, to determine if development of moment-to- moment activity and interactions between variables are altered in infants at risk. The temporal characteristics of sleep-walking states, including distribution of states across the night, state transition probabilities, periodic organization, and time of night modulation of sleep and waking physiology will be compared between control infants and infants at risk. Relationships of moment-to-moment physiological alterations in activity to the 3-4 h "feeding rhythm" and to time-of-night will be quantified for each group. Assessment of instantaneous variation in cardiac and respiratory intervals will include Poincare procedures and phase-plane plots, together with ANOVA assessment of dispersion at different rates. Multivariate statistical procedures, including stepwise logistic regression and discriminant analysis, will also be used to assess differences among risk groups.